Renewable energy is the key to future global sustainability, and many endeavours are being made to harvest renewable energy in an efficient and profitable manner. Environmental concerns and diminishing fossil fuel reserves increase the urgency of transitioning towards clean renewable energies. This explains the exponential growth of wind turbine (WT) and photovoltaic (PV) usage in the past few years. However, the variable nature of the power produced by WTs and PVs makes their controllability a challenge. Furthermore, when connecting renewable energies to the grid, increasingly stringent grid connection standards must be met. These standards emphasize the importance of fast control, the quality of the injected power into the grid, and robustness. Two main issues must be addressed in order to bring grid-connected renewable energies into the mainstream: cost and controllability.
A typical power conditioning system for renewable energy applications has two stages, and therefore requires two separate control schemes. The first stage is called the input-side converter, and is typically an AC/DC rectifier for WTs (see FIG. 1A) or a DC/DC converter for PVs (see FIG. 1B). The second stage is called the grid-side converter, and is typically a DC/AC inverter for most systems. The main task of the input-side converter is to achieve Maximum Power Point Tracking (MPPT) and extract maximum power from WTs or PVs. The main task of the grid-side converter is to ensure that all of the power extracted by the first stage is transferred to the grid quickly. To achieve this, the voltage across the capacitor between the two stages, also called the DC-bus capacitor, must be regulated. Although the second stage converter should also perform other important tasks, such as injecting high quality current to the grid, islanding detection, synchronization, reactive power compensation, and other ancillary services, regulating the DC-bus voltage is vitally important in order to guarantee the reliable operation of the power conditioning system.
The DC-bus capacitor acts as an energy storage capacitor and provides the flexibility to alternate the instantaneous power in between the two stages, giving the system the ability to absorb sudden changes in power coming from the input-side converter. More importantly, in single-phase power conditioning systems, the DC-bus capacitor is used to decouple the power ripple by providing a low frequency current. The main challenge regarding the DC-bus voltage loop controller for grid-connected single-phase inverters is the presence of the low frequency ripple in the DC-bus capacitor voltage (this ripple is also present in three-phase unbalanced systems). In a conventional DC-bus voltage control scheme, a very low bandwidth PI controller is usually used to regulate the DC-bus voltage. The PI controller should have a very low bandwidth in order to prevent the low frequency ripple from propagating to the control loop through the DC-bus voltage feedback. Because of this, the conventional DC-bus voltage controller produces a very sluggish and poor transient response. This not only affects the performance of the DC/AC converter, but also forces the designer to over-design the converter in order to have reliable performance against the overshoots/undershoots that occur during transients.
Based on the above, there is therefore a need for developments which mitigate if not overcome the shortcomings of the prior art.